de Bruijn  [1]} approximated \(\Phi (t)\)  with \(\Phi _{dB}(t)\) :
\(\aligned \Phi _{dB}(t)&=\exp \left(-2\pi \cosh t\right)\\&\times \left(4\pi ^2\cosh (t/4)+(4\pi ^3-6\pi )\cosh (5t/4)+4\pi ^2\cosh (9t/4)\right)\endaligned \) 
